Washington (CNN)Russian officials bragged in conversations during the presidential campaign that they had cultivated a strong relationship with former Trump adviser retired Gen. Michael Flynn and believed they could use him to influence Donald Trump and his team, sources told CNN.

The conversations deeply concerned US intelligence officials, some of whom acted on their own to limit how much sensitive information they shared with Flynn, who was tapped to become Trump's national security adviser, current and former governments officials said. "This was a five-alarm fire from early on," one former Obama administration official said, "the way the Russians were talking about him." Another former administration official said Flynn was viewed as a potential national security problem.

The conversations picked up by US intelligence officials indicated the Russians regarded Flynn as an ally, sources said. That relationship developed throughout 2016, months before Flynn was caught on an intercepted call in December speaking with Russia's ambassador in Washington, Sergey Kislyak. That call, and Flynn's changing story about it, ultimately led to his firing as Trump's first national security adviser.